The present invention relates to AC circuits, and more particularly, to a device for measuring the power that is coupled to a load.
To simplify the following discussion, the present invention will be discussed in reference to cellular telephones; however, it will become apparent from the following discussion that the present invention can be utilized in other applications. The power output by a cellular telephone is varied such that the signal received by the base station is more or less constant independent of the distance from the cellular telephone to the base station. When the cellular telephone is far from the base station, the output amplifier is set to generate the maximum allowed power. When the cellular telephone is close to the base station, the power level is reduced to a small fraction of the maximum allowed power. While the cellular power level can be controlled by a feedback loop relying on the signal received by the base station, such control is not always optimal. Accordingly, a device within the cellular telephone that can measure the actual power being output by a cellular telephone would provide advantages if the device did not draw a significant amount of power from the cellular telephone batteries.
The present invention is a meter for measuring the root-mean-squared potential of an AC signal characterized by a frequency f. The meter includes first and second capacitors. The AC signal is applied to the first capacitor, which includes first and second plates separated by a distance that depends on the root-mean-squared potential of the AC signal, but not on changes in the AC signal that occur over a time of the order of 1/f. The second capacitor has first and second plates separated by a distance that depends on the separation of the first and second plates in the first capacitor. A detection circuit measures the capacitance of the second capacitor. The first plate of the first capacitor is preferably connected to the first plate of the second capacitor by a non-conducting mechanical link. In the preferred embodiment of the present invention, the second electrode of the first capacitor includes a conducting layer on a substrate and the first plate of the first capacitor includes a conducting plate suspended over the first plate by a spring supported on the substrate.